Vinyl films plasticized with plasticizers have been used for many years in adhesive labels, tapes and decorative sheets. Vinyl films, particularly polyvinyl chloride (PVC) films, have had wide acceptance for such applications because, among other things, they are inexpensive and weather resistant and can be colored easily with pigments and dyes. In addition, plasticized polyvinyl chloride (PVC) has had particularly wide acceptance because its properties can be modified over a wide range by the incorporation of plasticizers. These films have been used in various graphic applications with success.
Although vinyl films have been useful in graphic and wall covering applications because of their superior flexibility and conformability, there is a continuing need to develop films which do not contain PVC. Halogen-containing materials, such as PVC, have generally been recognized as producing undesirable, environmentally unfriendly, by-products when burned.
Accordingly, there is a need for environmentally friendly, non-halogen containing, and in particular, non-PVC containing films that have properties that are comparable to films containing halogens. In some cases, PVC based vinyl materials are being replaced by polyolefins due to the relatively lower cost, many choices of available materials, and the flexibility of polyolefins. A key problem with polyolefin based material is its poor print quality. In replacing vinyl film with polyolefin films, attempts have been made to have a multilayer coextruded film using ink receptive skin layers. However, the print quality of these coextruded films is not good for inkjet printers.
A number of graphics applications, such as wrapping of vehicles including busses, trailers and the like, require the use of a conformable film such that a graphics installer can easily apply such a film to a non-smooth surface. To produce a PVC-free film material with good printability, one will typically use a polyolefin material such as a polypropylene or polyester substrate with a printable top coat. Polypropylene and polyester however are not considered to be “conformable” material and do not perform well on non-smooth surfaces. Further, more conformable base materials, such as polyethylene and ethylene vinyl acetate (EVA), are difficult to use in graphics applications due to lower modulus of the film especially under top-coat drying conditions.
Moreover, there is a need in graphics industry to print on media with superior gloss level. Although coextruded film can achieve a glossy surface by casting onto a polished roller, it is in general a difficult balance to select an extruded skin material with good ink adhesion property which is not tacky.
It is also, highly desirable, to have a graphics film with good dimensional stability throughout the life cycle of its use. Graphics films which tend to shrink over time will cause the adhesive edges of the film to become exposed after application. Traditionally, cast vinyl film has lower shrinkage versus calendared vinyl. Lower stress in the cast process results in a lower shrinkage and thus a more dimensionally stable film.
Many different approaches have been taken in recent years for producing decorative graphics films. These processes are generally categorized by solution casting techniques or extrusion techniques. For instance, U.S. Pat. No. 4,810,540 to Ellison et al. and U.S. Pat. No. 4,902,557 to Rohrbacher use solution casting techniques in which liquid-cast, solvent-based clear coats and pigmented base coats are applied to a flexible casting sheet by a coating process such as reverse roll coating or gravure printing. The liquid cast layers are separately applied and then dried at high temperatures to evaporate the solvents.
U.S. Pat. Nos. 4,317,860 and 4,364,886 to Strassel also disclose coextrusion of multi-layer films such as a two-layer coextrusion of predominantly PVDF on one side and a predominantly acrylic resin on the other side of the coextruded sheet. These unitary structures are used to make molded articles, or to adhere the sheets to a molded polymer.
Film extrusion techniques also have been used in the past for making free films in which the extruded polymeric material is coated on a polished drum. These films are then undercoated with various color coats. The exterior surface of the extruded free film that contacts the drum (and is separated from the drum as a free film) does not have a high gloss and high distinctness-of-image. Also films manufactured in this manner do not have a carrier sheet attached, which makes them hard to handle and easily damaged in subsequent processing.
The present invention, utilizes a series of process steps, to create a polyolefin based graphic film face-stock which possesses superior print quality, high gloss, and low stress buildup and good dimensional stability to satisfy the needs of the graphics industry.